This invention relates to wiper control devices for automobiles or the like, and more particularly to a wiper control device for allowing a wiper to perform an intermitting wiping operation.
In general, a wiper control device for an automobile, for instance, has a continuous wiping mode for driving the wiper continuously, and an intermittent wiping mode for driving the wiper intermittently. In the intermittent wiping mode, a relay switch provided in a wiper motor powering path is turned on and off periodically by an interval control circuit.
In the above-described conventional wiper control device, the relay switch with mechanical contacts is employed as a switching element for driving the wiper motor. Therefore, the sounds generated when the relay switch is turned on and off are relatively noisy.
In order to overcome these difficulties, similarly as in the case of the internal control circuit made up of solid state elements, it is desirable to use solid state elements to form the wiper motor driving switching element; that is, instead of the relay switch, a semiconductor switching element such as a power transistor should be employed. However, in this connection, it should be noted that in general a semiconductor switching element is liable to be broken by overcurrent, whereas, in an automobile, short-circuit current due to dead short or overcurrent due to the locking of the wiper motor would be caused. Hence, it is not practical to merely use solid state elements to form the wiper motor driving switching element. Furthermore, a semiconductor switching element is liable to be broken when thermal runaway occurs with it to abnormally increase its temperature. This difficulty may be eliminated by the following method: The actual temperature o of the semiconductor switching element is compared with a predetermined upper limit temperature, and the semiconductor switching element is forcibly turned off according to the result of the comparison. However, the method is still disadvantageous in the following point: If the semiconductor switching element is forcibly turned off merely according to the result of comparison, then chattering occurs with the semiconductor switching element.
The relay switch may be of a change-over type having make and break contacts. Therefore, in dynamically braking the wiper motor to positively stop the wiper at a predetermined position, the break contact can be utilized as it is. However, for the above-described utilization of solid state elements, it is necessary to additionally provide a semiconductor switching element for dynamically braking the wiper motor. Furthermore, if the semiconductor switching element for driving the wiper motor and the semiconductor switching element for dynamically braking it are merely combined with each other, then they may be turned on at the same time, thus short-circuiting the power source. Hence, it is rather difficult to use solid state elements to form the wiper motor driving switching element.